[SPARK-6843][core]Add volatile for the "state"

Fix potential visibility problem for the "state" of Executor

The field of "state" is shared and modified by multiple threads. i.e:

```scala
Within ExecutorRunner.scala

(1) workerThread = new Thread("ExecutorRunner for " + fullId) {
  override def run() { fetchAndRunExecutor() }
}
 workerThread.start()
// Shutdown hook that kills actors on shutdown.

(2)shutdownHook = new Thread() {
  override def run() {
    killProcess(Some("Worker shutting down"))
  }
}

(3)and also the "Actor thread" for worker.

```
I think we should at lease add volatile to ensure the visibility among threads otherwise the worker might send an out-of-date status to the master.

https://issues.apache.org/jira/browse/SPARK-6843

Author: lisurprise <zhichao.li@intel.com>

Closes #5448 from zhichao-li/state and squashes the following commits:

a2386e7 [lisurprise] add volatile for state field

core/src/main/scala/org/apache/spark/deploy/worker/ExecutorRunner.scala

@@ -50,7 +50,7 @@ private[deploy] class ExecutorRunner(
     val workerUrl: String,
     conf: SparkConf,
     val appLocalDirs: Seq[String],
-    var state: ExecutorState.Value)
+    @volatile var state: ExecutorState.Value)
   extends Logging {
 
   private val fullId = appId + "/" + execId